1. Field of the Invention
The field of art to which this invention relates is cryptographic communication. Specifically, this invention provides a method based on physical principles for secretly distributing two sets of binary encryption keys that can be used to encrypt publicly transmitted messages between two parties.
2. Description of the Related Art
In general, to establish a secret channel between two parties and two parties only, there are three possible solutions. The first method is to use a secret courier who can deliver the message with secrecy. The second method involves the case, that is referred to as the xe2x80x9cPublic Key.xe2x80x9d In this case party A and party B publicly establish a mutual agreement over two prime numbers p and q. Party A then chooses a secret number x and publicly transmits a public number px (mod q) to party B. Similarly, party B chooses a secret number y and transmits a number py (mod q) to A. Party A then computes the number (py)x=px.y (mod q) and Party B computes the number (px)y=px.y (mod q). Using this method, a mutually identical key can be established. The secrecy in this method is guaranteed only by the assumption that a third party does not possess the computing power to factorize the numbers. Both the first and second methods are well known in the art.
The third method is often referred to as xe2x80x9cQuantum Cryptography.xe2x80x9d The basic principle of operation for xe2x80x9cQuantum Cryptographyxe2x80x9d can be summarized as follows. Sender A prepares a twin-particle quantum mechanical state. Such a state consists of two and only two quantum mechanical particles (x and y) (e.g., photons). The state is prepared in such a way that they fall into the general class of xe2x80x9cEntangled Quantum States.xe2x80x9d Such a state possesses the property that the behavior of particle X is closely related to that of particle y. For example, if one prepares such a state and measures whether photon x is left or right-hand polarized. The result is closely related to the result if one were to perform a simultaneous measurement of such properties on particle y. In a special case (referred to as the Einstein-Podolsky-Rosen (EPR) state), the handiness of the polarization of the particles x and y are always opposite.
After preparing the entangled two-particle quantum state, the sender (A) sends one particle (x) through a channel to a receiver (B). The receiver at the right moment after receiving the particle (x), decides to rotate its polarization by 90xc2x0 (denoting a binary xe2x80x9c1xe2x80x9d) or do nothing (denoting a binary xe2x80x9c0xe2x80x9d) and send the particle (x) back to the original sender (A). Upon receiving the particle (x) back from B, the original sender (A) can perform two identical measurements on both particles x and y, using a variety of polarization bases. If the outcome of the two measurements are the same for both particles (x and y), the sender (A) can conclude that the receiver (B) replied to the sender (A) a binary number xe2x80x9c0xe2x80x9d. If the outcome of the two measurements are rotated by 90xc2x0, then a binary number xe2x80x9c1xe2x80x9d is registered. Since there is only one quantum x (e.g., a photon) that is sent at a time when one bit of a secret key string is communicated, if the photon (x) is captured or tampered with by an eavesdropper (C), the polarization properties of the photon will be lost. Hence the method is safe from eavesdropping.
Prior art schemes which utilize Quantum Cryptography use laser sources instead of a single photon pair source, and therefore cannot be considered a true quantum cryptographic communication channel. While these schemes have their advantages, they are plagued by the following disadvantages:
1. The prior art schemes do not provide a secret communication channel between two and only two parties by using a single photon to carry the binary key string information, hence, they do not preserve secrecy based on physical principles;
2. The prior art uses a single particle""s polarization entanglement state which requires one of the two entangled particles to travel through the distance between the two communicating parties twice, during this long distance, any disturbance to the pathway channel (i.e., thermally or mechanically induced birefringence) obstructs the polarization of the communication channel and introduces error;
3. The prior art uses a single particle""s polarization entanglement state which is prone to naturally occurring birefringence, which can also obstruct the communication channel and introduce error; and
4. The prior art uses a phase modulation for communication which is required to be preserved for twice the long communication pathway length which is particularly prone to external disturbance (i.e., thermal or acoustic disturbances that are fast enough to cause an inhomogeneous change to the pathway (fiber channel) length during the entire communication period), again this affects the communication channel and introduces error.
The present invention resolves all of the above problems by communicating through a conventional pathway channel using the quantum coherence properties between two single photon sources, and in particular is based upon the physical principle that the quantum mechanical state of a single quantum, if unknown, cannot be copied.
Accordingly, a quantum cryptographic communication channel is provided. The quantum cryptographic communication channel comprises: a light source; directing means; first and second sources each capable of generating a pair of photons emitted in the form of signal and idler light beams when energized by the light source, the first and second sources being arranged relative to each other such that the idler beam from the first source is incident upon the second source and aligned into the idler beam of the second source and the signal beams are directed by the directing means to converge upon a common point; a light modulator for changing the phase of the idler beam from the first source between first and second phase settings before being incident upon the second source; a controller for controlling the timing of the phase change from the first phase setting to the second phase setting; first and second detectors for detecting the incidence of the signal beams from the first and second sources; and a beam splitter disposed at the common point for directing the signal beams to the first detector when the phase of the idler beam from the first source has the first phase setting and to the second detector when the phase of the idler beam from the first source has the second phase setting.
In a preferred embodiment of the present invention, the detection of the signal beams at the first detector corresponds to a first logical value and the detection of the signal beams at the second detector corresponds to a second logical value wherein the controller controls the timing of the phase change from the first phase setting to the second phase setting corresponding to the first and second logical values, respectively, to thereby transmit a cryptographic key string comprising a plurality of the first and second logical values.